(1) Field of the Invention
The present invention generally relates to an electrolytic capacitor including an anode foil, a cathode foil and an electrolytic paper impregnated with electrolyte, which paper is interposed between these foils. More particularly, the present invention relates to an improvement in such electrolytic paper whose swelling degree for the electrolyte is extremely increased to improve the equivalent series resistance (ESR) of the electrolytic capacitor without generating short-connection troubles.
(2) Description of the Prior Art
Generally, an electrolytic capacitor, particularly an aluminum electrolytic capacitor, is produced by following process. An electrolytic paper is interposed between an anode foil and a cathode foil, and they are wound up. They are immersed in an electrolyte which penetrates the electrolytic paper, and finally their ends are sealed. The electrolyte for this electrolytic capacitor is a solution of ethylene glycol, dimethyl formamide, or the like as solvent and boric acid, ammonium adipic acid, ammonium hydrogen maleate, or the like as solute. The electrolyte penetrates the capacitor element from both of its ends.
The density and thickness of the electrolytic paper has previously been determined to reduce the generation of short-connection troubles during the winding of the capacitor element. Generally, in order to reduce the generation of short-connection trouble during the winding process, the thickness and density of the electrolytic paper are increased. However, its impedance property, especially equivalent series resistance (hereinafter, referred to "ESR"), will become large as the thickness and density are increased. ESR will be primarily worsened in response to the increased thickness, and secondarily worsened in response to the increased density. On the contrary, the density and thickness of the electrolytic paper should be decreased to lower ESR.
The value of CSF according to JIS P8121, representing the degree of pulp resolution, does not affect ESR, but the density is increased in inverse proportion that the value of CSF is lowered. Thus it is difficult to effectively decrease ESR without generating short-connection trouble.
When ethylene glycol is used for the solvent of the electrolyte, this solvent worsens the electrical properties of the electrolytic capacitor at low temperatures, because ethylene glycol has a high viscosity. When dimethyl formamide is used for the solvent of the electrolyte, this solvent improves the electric properties at low temperature but this solvent is remarkably toxic. In recent years, to overcome these problems, .gamma.-butyrolactone has been used for the solvent of the electrolyte. The electrolyte containing .gamma.-butyrolactone has low viscosity and low toxicity, thereby improving the electric properties at low temperature and workability.
However, such constituted conventional electrolytic capacitor has a large ESR value rather than a limited value. To avoid this problem, the thickness and density of the electrolytic paper are kept within a specific range which prevents the generation of the short-connection trouble during winding process. After the electrolyte impregnation, the fibers of the electrolytic paper will be expanded and the space between the fibers will be extended if the degree of swelling of the electrolytic paper is remarkably increased by the electrolyte in comparison with conventional swelling. This will essentially decrease the density of the electrolytic paper, and thus ESR will be decreased. The short-connection troubles will not be further generated after the winding work.
In conventional arts, however, ESR has not been improved by the impregnation of electrolyte containing ethylene glycol, dimethyl formamide, or the like as the solvent and conventional arts do not provide effective means for reducing ESR. Although the electrolyte containing .gamma.-butyrolactone can improve the electrical properties at a low temperature and workability owing to its low viscosity and low toxicity, it worsens ESR. Since the electrolyte containing .gamma.-butyrolactone has a poor hydrophilic property in comparison with the other conventional electrolyte, the cellulose fiber of the electrolytic paper is not swelled after swelling process. Thus the density of the electrolytic paper is not decreased. Particularly, the electrolytic capacitor for voltages of 50 V or more is desirably made of an electrolytic paper having a density of 0.6 g/cm.sup.3 or more, in order to reduce the generation of short connection trouble and improve its pressure resisting property. The fibers of such high density electrolytic paper are tightly engaged with each other through their hydrogen bonds, and are not swelled by the electrolyte containing .gamma.-butyrolactone so that such type electrolytic capacitor increases ESR. In order to overcome these problems, conventional electrolytic capacitors have included a mixture of .gamma.-butyrolactone, ethylene glycol, dimethyl formamide, and/or water, and an electrolytic paper having as low a density as possible. But they have satisfactorily decreased the ESR.